The invention is in the field of medical ultrasound apparatus, particularly apparatus for use in ultrasonography of the eye.
Ultrasound may be used in a variety of medical applications, including diagnostic ultrasonography of the eye. Diagnostic information is typically provided by an ultrasound pulse from a piezoelectric transducer, which is directed into a tissue. Reflected acoustic energy is detected (as xe2x80x98echoesxe2x80x99), so that the amplitude of the received energy may be correlated with the time delay in receipt of the echo. The amplitude of the echo signal is proportional to the scattering strength of the refractors in the tissue, and the time delay is proportional to the range of the refractors from the transducer. A variety of hand-held ultrasound instruments for measuring corneal thickness (called pachymeters) have been developed (for example see U.S. Pat. Nos. 4,564,018; 4,817,432; 4,930,512). Many prior art ultrasonic pachymeters provide A-scan output, in the form of waveforms displayed on a cathode ray tube, representing acoustic reflections in a single dimensional xe2x80x98columnxe2x80x99 of tissue.
In B-scan ultrasonography, a two-dimensional image is formed, in which pixel brightness reflects the amplitude of the reflected acoustic signal. A B-scan image therefore represents a cross-sectional slice of the imaged tissue. The cross-sectional information is typically provided by correlating information from a series of adjoining columnar scans (each of which may be used to produce A-scan output). For the purpose of producing B-scans, adjoining columnar scans may be produced by a number of methods: rectilinear translocation of a transducer over the tissue of interest; pivoting angular displacement of a single transducer over a fan-shaped area; or through the use of a linear array of transducers.
In some applications, three dimensional images may be reconstructed from a series of B-scans. U.S. Pat. No, 4,932,414 to Coleman et al. for example describes a system in which the transducer is electronically swept or physically rotated to produce a series of sectored (fan-shaped) scan planes which are separated by a known angular distance, to produce a 3-dimensional display. In a similar fashion, U.S. Pat. No. 5,487,388 to Rello et al. discloses an ultrasonic scanning system in which sequential fan-shaped B-scan image planes are obtained by movement of the transducer probe in an arc, a movement which allows the apex of the scanned 3-dimensional volume to be located below the probe to facilitate imaging between closely-spaced surface obstructions.
The structure of the eye, particularly the cornea, presents special problems for optimal ultrasonographic B-scan imaging. The human cornea is an asphere, flattening concentrically, typically approximately 11 mm across with an average central radius of curvature of 7.8 mm which increases towards the periphery. The high resolution required for ultrasonic imaging of some corneal structures is optimally achieved if ultrasound data is collected from the focal point of the transducer, and the ultrasound beam is normal to the surface of the cornea. As a result, rectilinear scanning of the cornea provides optimal imaging information only from relatively small segments of the cornea which are normal to the transducer beam and in the plane of beam focus. Similarly, volumetric 3-dimensional scanning by reconstruction of a series of fan-shaped B-scan planes, as for example described in U.S. Pat. Nos. 4,932,414 and 5,487,388, is not a system adapted to provide the degree of resolution required for biometry of the corneal surface.
High frequency ultrasound has been used in ophthalmological ultrasonography to obtain biometric B-scan images of the human cornea, by arcuate translocation of a single element focused transducer. Silverman et al., 1997, J. Ultrasound Med. 16:117-124, describe a system for sonographic imaging and biometry of the cornea in which a sophisticated programmable motion system permits ultrasonographic arc scanning. In the Silverman et al system, the ultrasonic transducer is translated through an arc matched to the approximate radius of curvature of the cornea using five servo motors and a controller. Similarly, U.S. Pat. No. 5,331,962 discloses an ultrasound system for corneal arc scanning, in which a transducer is translocated along a curved track that approximates the surface curvature of the cornea
In one aspect of the invention, an apparatus for ultrasound scanning of the eye is provided comprising a virtual center translocation mechanism that facilitates precise arcuate motion of an ultrasonic transducer to maintain focal distance from the eye and to maintain normality of the ultrasound beam with surfaces of the eye. The arcuate movement of the transducer focal path may closely approximate the surface of the cornea. Some embodiments of the invention may include a radius adjust mechanism for changing the radius of ultrasound scanning, to accommodate different eye sizes and to facilitate positioning of the ultrasound transducer focal point on selected surfaces of the eye, such as the cornea. Centration optics may also be provided, for aligning the translocation path of the ultrasound transducer with an axis such as, but not limited to, the Purkinje axis of a patient""s eye.
In one embodiment, the invention provides an ultrasound transducer support comprising a transducer mount adapted to accommodate an ultrasound transducer having a focal point. The support may be provided with a virtual center mechanism attached to the transducer mount, for moving the ultrasound transducer along an arcuate translation path. The arcuate translation path of the transducer may be offset from a virtual center of translocation by a radius of transducer translocation, so that the focal point of the ultrasound transducer traverses an arcuate focal path about the virtual center of translocation. A radius adjust mechanism may be provided for adjusting the position of the transducer mount to change the radius of transducer translocation.
In an alternative embodiment, the invention provides a method of ophthaamological ultrasonography comprising centring an ultrasound transducer having a focal point in alignment with the Purkinje or other optical or geometric axis of a patient""s eye using centration optics, and moving the ultrasound transducer along an arcuate translation path intersecting the Purkinje or other optical or geometric axis of the patient""s eye. The arcuate translation path of the transducer may be offset from a virtual center of translocation by a radius of transducer translocation, so that the focal point of the ultrasound transducer traverses an arcuate focal path about the virtual center of translocation.